1. Technical Field
This invention relates to the art of thermally spraying metals, and more particularly to spraying metals with solid lubricant particles by cored wires.
2. Discussion of the Prior Art
Thermal spraying was initiated as early as 1910 when a stream of molten metal was poured into the path of a high pressure gas jet causing metal droplets to spray in a conical pattern onto an adjacent substrate to immediately freeze and form a coating of deformed particles in a lamellar structure. Today, there are essentially two types of thermal spraying that use wire feedstock: combustion flame spraying and electric arc spraying. In the combustion flame process, wire is fed continuously into an oxygen-fuel gas flame; high temperatures are generated after mixing with the oxygen and igniting the flame. Compressed air is directed to the molten tip of the feedstock wire to atomize and project the metal particles. In general, coatings produced by the combustion flame process are relatively high in oxides and high in porosity levels, and, due to the low particle velocity (e.g., 50-100 m/sec), adhesion strength is relatively low at 5-20 MPa.
In the electric arc process, an electrical arc is struck between two wires, or, in some cases, one wire and an accompanying anode, the wire serving as a consummable electrode. The arc continuously melts the wires and compressed air is blown directly behind the point of melting to atomize and project the molten droplets to a target substrate. The droplets deform on impact and form a more adherent coating due to higher particle velocities of 150-300 ms.sup.-1. The oxide level is medium to low and the coating exhibits overall lower porosity than flame-sprayed coatings.
It is known to use a cored steel wire feedstock, filled with wear-resistant producing ingredients and a minor proportion of graphite, to function as a consummable electrode in electric arc welding (see U.S. Pat. No. 4,071,734). It is also known to arc-spray cored steel wire feedstock, filled with hard carbide particles or CrBSi (see "Arc Spraying of Cored Wires", K-H. Busse, SPRAYTECH GmbH, FRG, Internal Proceedings of Thermal Spray Technology, June, 1989, Paper 36, pages 19-28). However, such wire feedstock is not suitable for use in thermal spraying of solid lubricant particles because essentially all the cored ingredients dissolve in the melted wire forming an alloy that does not possess lubricity and because such cored ingredients (carbonates, fluorides, carbides, silicates) are undesirable for the purposes of this invention. Thermal sprayed coatings of a composite material have also been accomplished by forming the entire feed wire of a metal matrix composite such as aluminum containing fibrous or particulate TiO.sub.2, Al.sub.2 O.sub.3, SiO.sub.2, Zr.sub.2 O.sub.3, SiC, or Si.sub.3 N.sub.4 (see U.S. Pat. No. 4,987,003). But such technique fails to provide deposition of discrete solid lubricant particles in a metal matrix.
Solid lubricants, particularly graphite, are difficult to dispense and integrate to an independent molten metal body without dissolution. Adding such graphite powder to the flame or arc-spray process, either upstream or downstream of the location where the wire melts, may not necessarily result in the intended graphite concentrations in the coating and, further, may fail to minimize ablation of the graphite as it is exposed to the projecting gases or molten metal.